426 C. Reynaud et al. International Journal of Refractory Metals Hard Materials 19(2001)425-435 On the other hand, pores left after burnout may be filled Table 2, and are compared with compositions reviewed by the liquid phase. The effects of size, content, type of in the literature; the organics contents are significantly 'A on tape casting processing, sintering characteris- lower in our case. The volume fraction of PFA incor tics, porosity control and microstructure are presented porated in the slurry is referred to the theoretical volume of the dry gree e ceramic volume tape casting organic volume PFA volume) 2. Processing and experimental procedures Two different types of PFAs: polyamide powders ( PAl2, Orgasol Elf-Atochem-France) and corn starch A fine alpha silicon carbide powder has been chosen (Roquette-France), with a spheroidal shape and a nar- (Sika Tech FCP13, Norton-Norway). Alpha polytype row particle size distribution have been used. PFA was preferred to beta polytype to avoid excessive grain characteristics are listed in Table I growth during beta-alpha transformation. An amount The incorporation of polymer particles in the suspen- of 5 wt%(related to all ceramic powders) of the YAG- sion without precautions has led after casting to the alumina eutectic(60 wt% Al2O3-40 wt%Y203) com- elaboration of brittle and less flexible tapes. Three strat position was added. Characteristics of raw materials are egies have been envisaged to take into account the Pfa listed in Table 1 () Volume compensation: a part of ceramic volume Tape casting slurries have been elaborated in four was replaced by the PFA. Consequently the volume tages. In the first stage, ceramic powders were mixed, fraction (ceramic PFA)versus organic additives by using ball milling alumina media, during 4 h in a volume was constant [4]. This method led to a high MEK-ethanol azeotrope solvent(60 vol% butanone-2/ nts: the tapes we 40 vol% ethanol) containing 0.6 wt% of a phosphate ened. Therefore, the pyrolysis of organic components ester as dispersant (Beycostat C213, CECA-France) was too problematic(cracking and swelling) accord Then(second stage), an acrylic binder and a phthalate ing to Corbin and Apte [12] plasticiser were added to the suspension and mixed (i) Weight compensation: the weight ratio between during 14-16 h. The third stage corresponds to the in- (ceramic+ PFA) and organic additives was kept con- A s oration of the PFA into the slurry mixing time of stant [12]. A cracking behaviour was obtained during h leads to an uniform distribution of the Pfa. fi the drying stage nally slurry was de-aired at a slow rotation speed during (ini) No compensation: the organic content was inde- 24h. Typical compositions of suspensions are given in pendent of the PFA. This method was retained. The Table I Characteristic parameters of the ceramic powders and of the different PFAs incorporated into the slurry Raw ceramic materi P-SiC FCP13 Alumina Cr15 Yttrium oxide dso(um)a 0.3 BET S(m/g) PFA PA-A PA-B PA-C Corn starch Chemical nature olyamide-I Polyamide-12 Polyamide-I Starch .5 122 Aspect rati 1.37 1.29 a Laser grain sizer analysis. Data obtained by image analysis. Table 2 Concentration of components in the dry tape for our tapes compared to compositions reviewed in the literature Refs Our study Corbin and Apte [12] Corbin and Apte [12 Davis et al. [4] Volume fraction of PFA referred 0.55 0 to ceramic volume Ceramic volume fraction"(vol % 68.4 37 38.7 31.5 fraction(vol % PFA volume fraction"(vol % 0 The PFA volume fraction incorporated is referred to the total volume composed of ceramic material+ pore forming agent(PFA). The volume fractions are referred to the theoretical volume of the dry green tape (i.e. ceramic volume tape casting organic volume PFA volume)2             M   &    0   /     
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